Recently there has been considerable interest within the detergents industry in the production of detergent powders having a relatively high bulk density, for example 550 g/l and above.
Generally speaking, there are two main types of processes by which detergent powders can be prepared. The first type of process involves spray-drying and aqueous detergent slurry in a spray-drying tower. In the second type of process the various components are dry-mixed and optionally agglomerated with liquids, e.g. nonionics. The latter kind of process is more suited to the production of powders having a relatively high bulk density. That is primarily because the chemical composition of the slurry used in the spray drying process markedly affects the bulk density of the granular product. This bulk density can only be significantly increased by increasing the content of relatively dense sodium sulphate. However, sodium sulphate does not contribute to detergency, so that the overall performance of the powder in the wash is thereby reduced.
One dry-mix process suitable for production of relatively high density products is described in European Patent Specification EP-A-0 420 317. This involves reacting a liquid acid precursor of an anionic surfactant with an alkaline inorganic material in a high-speed mixer/densifier, treating the material in a moderate-speed granulator/densifier, and finally drying and/or cooling the material. The heat of the neutralization reaction between the acid surfactant precursor and the alkaline material is used to bring the starting material into a deformable state, and results in densification of the detergent composition.
In the case of powders which also contain a nonionic surfactant, it is possible to "structure" the (liquid) nonionic by reacting the acid precursor and the alkaline material in situ i.e. by dissolving the precursor in the nonionic and then adding the alkaline material to the solution in the first stage of the process. The same structuring can be achieved by in situ formation of a soap during the first stage, i.e. substituting a fatty acid for the anionic acid precursor so that the soap is formed by a saponification reaction during that stage.
One drawback of such a process is the poor i.e. broad particle size distribution of the resultant powder. This can be conveniently expressed by two measures:
(a) The total amounts of fines (&lt;180 microns) and coarse (&gt;1400 microns) in the product. PA1 (b) The n value of the Rosin Rammler distribution. This is calculated by fitting the particle size distribution to an n-power distribution according to the following formula: ##EQU1## where R is the cumulative percentage of powder above a certain size D. D.sub.r is the average granule size and n is a measure of the particle size distribution. D.sub.r and n are the Rosin Rammler fits to a measured particle size distribution. PA1 (i) forming a liquid feedstock comprising a liquid binder and a structurant; PA1 (ii) dosing the liquid feedstock and a solid component into a high-speed mixer/densifier, to form a granular detergent material and forming or introducing further structurant in the high-speed mixer/densifier; PA1 (iii) subsequently treating the granular detergent material in a moderate-speed granulator/densifier, whereby it is brought into or maintained in a deformable state; and PA1 (iv) drying and/or cooling the product of step (iii).
A high n value means narrow particle size distribution and low values mean a broad particle size distribution.
Typically powders produced by the aforementioned kind of granulation process have a total coarse and fines levels of around 20%. This usually translates into n values around 1.5. This is a problem in processing, since fines need to be recycled and coarse granules may need milling. Further since between the range 180-1400 microns, the particle size distribution is broad, the powders may have a negative impact on consumer product perception. Particularly excessive levels of fines can lead to poor dispersion/dissolution characteristics in use. This is due to a tendency for the powder bed to gel on contact with water in the wash, which in turn subtracts from the total wash performance. It also leaves undesirable residues and causes negative interaction with sensitive fabrics.